On*tol"o*gy (?), n. [Gr. ? the things
which exist (pl.neut. of ?, ?, being, p. pr. of ? to be) + -
logy: cf.F. ontologie.] That department of the
science of metaphysics which investigates and explains the nature and
essential properties and relations of all beings, as such, or the
principles and causes of being.